How Does the Acorn Get from Here to There? – Scrub Jays and Oak Trees

With a few exceptions trees in the Oak genus (Quercus) are easily, if not immediately, recognizable.  There are approximately 600 species in the genus divided into two sub-genera.  Oaks are found in North and South America, Europe, North Africa, and Asia.  The oaks in Asia are in the sub-genera, the Ring-Cupped Oaks (Cyclobalanopsis), whereas oaks in the rest of the world are members of the Quercus sub-genera.

Oaks have complicated relationships with a number of other species ranging from symbiotic fungus to parasitic wasps to humans.  Oaks feature in our mythology, we use the bark of Quercus suber, Cork Oak, to make stoppers for wine and for soft flooring, we make furniture and barrels from some species of oak, we made cart and early car axles from particularity strong species, they make excellent firewood, and they are fun to climb.

Climbing a Coast Live Oak (Quercus agrifolia) when I was little

Oaks also make acorns.  Sometimes, particularly when mast fruiting, oaks produce enormous quantities of acorns.  Most of these acorns are eaten by animals; insects, humans, pigs, squirrels, birds, and a host of other animals.  The survival and reseeding rate for acorns is low, but oak trees tend to be long-lived, some reaching ages of 500 years or more.  In the absence of other factors this low seedling success rate is not an issue as the tree produces thousands of acorns each year for hundreds of years.  Some seeds are bound to survive and turn into new trees.

Oaks have a particular problem.  Their seeds (acorns) are large.

Coast Live Oak (Quercus agrifolia) acorns

By themselves the trees can only drop the acorns under their own drip-line, in the shade where they will not sprout.  How does the tree send its seeds to a new place where they can sprout and are not left in a dense mat of easily found and eaten food?

Plants, being clever and manipulative in their slow vegetative manner, have all manner of methods for getting animals to carry their seeds far and wide.  Oaks harness many species to do this work, bribing them with the highly nutritious seeds they produce.  Across much of North America scrub and blue jays are put to work distributing acorns across the landscape.

Western Scrub Jay (Aphelocoma californica) in the Santa Monica Mountains – possibly Belding’s Scrub Jay (Aphelocoma californica obscura)

Meet the Western Scrub Jay (Aphelocoma californica), also known as the California Scrub Jay, and sometimes known as, “That damned bird!”  It is a mid-sized bird, perhaps a foot long including tail, loud, strong, clever, and imperious.  Like all jays it is in the Crow family (Corvidae), one of, if not the, smartest of bird types.  Corvids are renowned for their problem solving abilities and feats of memorization.  Scrub jays are no exception.

When the acorns are ripe jays congregate on the trees, grab as many acorns as they can, and fly off to stash them for future use.

Scrub Jay carrying acorns to hide for lean times

Each bird seems able to carry 3 or 4 acorns at a time, in the picture above there are two in the jay’s beak and at least one more in its crop.

Jays will carry acorns up to a mile and a half, hiding them in widely distributed caches of 1-3 acorns per cache.  The bird memorizes the locations of each cache, that of any other caches it sees other birds store, and will move its own caches if it knows it has been observed making its own cache.  Some of these caches will be forgotten and in some of those the seeds will sprout.

One bird doesn’t seem like it would make much of an impact, but one must recognize both the diligence of each bird and the number of birds engaged in this activity.

Scrub Jays harvesting acorns (@ 40 photographs taken over @ 10 minutes)

The photo above is a compilation of about 40 photographs taken over roughly 10 minutes.  This level of activity has been constant on this tree throughout the day over the past 2 or 3 weeks.  The scale of the endeavor starts to become apparent.  Beneath the tree ground squirrels and gray squirrels gather seeds from the ground to add to their own larders as well.

The oak tree has effectively expanded its dispersal distance from a few feet to over a mile.  Not only that, the oak tree has found a way to have its seeds hidden in safe locations and planted in the ground.  Only a small proportion of the acorns will survive to make new trees, but over the 350 year expected life-span of this particular tree it is not unreasonable that several hundred acorns will survive to produce trees that will live long enough to produce seeds of their own.

Scrub Jay enjoying the sun

+++ Cathy commented that any discussion of oak trees in California is incomplete if Acorn Woodpeckers (Melanerpes formicivorus) are not included.  They don’t live where I am at the moment, but last week I was up in my old stomping grounds and visited one of my favorite grainery trees.  Grainery trees are where these communal woodpeckers store and dry their collected acorns.  This particular tree is an ancient, wind-blasted Douglas Fir atop Mt Tamalpais, has a nearly 4 foot diameter, has been lightening struck numerous times, and sits amidst a copse of large moss enshrouded oak trees.

Old grainery trees will be used by many generations of these little woodpeckers and the trees look like an art project .

In any event, here is a photo of part of a grainery.+++

Acorn Woodpeckers (Melanerpes formicivorus) use all surfaces of a tree to make their larders. They will use fence-posts and the sides of barns as well.

*   *  * *** *  *   *

As a final note, in many areas, but in California in particular, oaks of all species are severely imperiled.  Oak woodlands are often considered to be the most important ecosystem in the region, but they have been subject to a number of stresses.  Oaks have been extensively cleared for orchards, vineyards, farmland, and urban use.  Saplings are eaten by cattle in range-lands, non-native feral pigs sniff out and eat all the acorns they can find, sometimes damaging tree roots in the process, and an ill-considered introduction of turkeys to the state by Fish & Wildlife to raise hunting revenue has led to even more acorns consumed by these overly prolific birds.

On top of all this, Phytophthora ramorum, a pathogen in the fungus-like family of water-molds, was accidentally introduced to the state via exotic ornamental plants and is causing wide-spread devastation.  This is commonly called Sudden Oak Death Syndrome and foresters strongly recommend not transporting oak firewood and washing cutting tools and boots when moving between oak growing regions.

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6 comments on “How Does the Acorn Get from Here to There? – Scrub Jays and Oak Trees

  1. cathybell says:

    No post about California’s oaks would be complete without a mention of acorn woodpeckers! I love these raucous, ever-cheerful birds, with their clownish expressions and wokka-wokka-wokka calls.

  2. Jean Flanagan says:

    Love the compiled photo – great way to tell a story.

  3. larooby says:

    Thanks for the effort and for sharing your insights and pictures. We live in the LA area and have recently completed all segments of the Backbone Trail, a great way to start to appreciate the profile of the fragile ecosystem this part of the world has to offer. Without your depth of knowledge, but with great enthusiasm, my partner posts at http://rosannefreed.wordpress.com/?s=backbone+trail

    • EarthKnight says:

      Looks like you’ve had fun on the Backbone Trail. I’ve only been on small portions of it.

      There are a number of great places to explore around LA, surprising considering how many people there are in the area and the size if the urban area.

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